Magnesium borohydride has been the subject matter of reports since about 1950. The preparative methods for magnesium borohydride up to about 1966 are discussed in a report titled, "Chemistry of Boranes IV. On Preparation, Properties, and Behavior Towards Lewis Bases of Magnesium Borohydride" by J. Plesek and S. Hermanek, appearing in Collection Czech. Chem. Comm/Vol. 31/(1966). These earlier reports (listed below) lead one to believe that the compound is now generally available, and comparable to sodium and lithium borohydrides possessing pronouncedly a salt like character (reports 2-6).
1. Barbaras G. D., Dillard C., Finholt A. E., Wartik T., Wilzbach K. E., Schlesinger H. I.: J. Am. Chem. Soc. 73,4585(1951).
2. Wiberg E., Bauer R.: Naturforsch. 5b, 397 (1950).
3. Wiberg E., Bauer R.: Z. Naturforsch. 7b, 58 (1952).
4. Wiberg E., Bauer R.: Chem. Ber. 85,593 (1952).
5. Wiberg E.: Angew. Chem. 65,16(1953).
6. Schrauzer G.: Naturwissenschaften 42,438 (1955).
This is not the case, however, according to J. Plesek, et al. who say "What papers used to call magnesium borohydride--that fact has now been established--were, at best, its adducts with various Lewis bases, or, in a few cases, mere solutions, in which borohydride anions and magnesium cations may be analytically proved. As the case stands, magnesium borohydride has not been hitherto dealt with as a chemical individuum."
J. Plesek, et al. also state in the above report that they found solvates of magnesium borohydride to be more or less readily prepared by all the routes reported, in varying degrees of purity. There is, however, one method only, which they found to afford desolvated magnesium borohydride, namely the following one accounted for by the equation: EQU MgH.sub.2 +B.sub.2 H.sub.6 +n(C.sub.2 H.sub.5).sub.2 O.fwdarw.Mg(BH.sub.4).sub.2 n(C.sub.2 H.sub.5).sub.2 O.fwdarw.Mg(BH.sub.4).sub.2 +n(C.sub.2 H.sub.5).sub.2 O
The adduct of magnesium borohydride with diethyl ether has no definite stoichiometric composition at room temperature. Most conveniently, it is prepared from magnesium hydride (synthesized by the direct combination of elements), and compressed diborane, by allowing the two reactants to stand in diethyl ether at room temperature. Description of properties of the ethereal solution of magnesium borohydride, of its desolvation, and of properties of the desolvated magnesium borohydride, is given under Experimental in the above report. Maximum attained purity amounted to 98 percent.
More recently, the preparation of ether soluble Mg(BH.sub.4).sub.2 in an 85 to 90% yield has been reported by V. N. Konoplev, Russian J. Inorg. Chem. 25(7), 964 (1980). The unsolvated compound is obtained, purportedly, by vacuum pyrolysis of the intermediate liquid dietherate Mg(BH.sub.4).sub.2.2(C.sub.2 H.sub.5).sub.2 O (MBDE).
The magnesium borohydride described hereinabove is required to be transformed to a stable form or to be prepared by another method if it is to have the desired properties of long-term thermal stability as required for a solid gas generator of H.sub.2 or D.sub.2 for laser applications.
Therefore, an object of this invention is to provide an adduct of magnesium borohydride which has the desired chemical and physical properties for long-term thermal stability.
Another object of this invention is to provide a method for preparing adducts of magnesium borohydride which possess long-term thermal stability.
A further object of this invention is to provide a method for preparing high purity adducts of magnesium borohydride which possess long-term thermal stability and compatibility with an oxidizer salt and binder in a solid gas generator for H.sub.2 and D.sub.2.